This is a thoracic vertebra as indicated by the joint surfaces for the ribs. These joint sufaces are smooth which suggests a terrestrial mammal (including seal); in dolphins and whales these articulations tend to be pitted. In animals such as deer there is a long spine on the back of each thoracic vertebrae and although the end of the spine is broken off I favour the short spine of a Seal. I have measured a common seal thoracic vertebra, which is very simillar to this one, and it is 7.6cm across so my best guess is a Grey Seal.